1. Field of the Invention
The present invention relates to a method of designing a semiconductor integrated circuit such as an MMIC (microwave monolithic integrated circuit) that operates in a radio frequency range, as well as to a semiconductor device designed by such a designing method.
2. Description of the Related Art
First, a conventional semiconductor device designing method will be described with reference to FIGS. 8A-8C. FIG. 8A is a design diagram of an amplifier. In FIG. 8A, reference numeral 101 denotes a transistor; 102, a matching circuit; 103, an input terminal of the amplifier; and 104, an output terminal of the amplifier. Power that is supplied to the input terminal 103 of the amplifier is amplified stage by stage by the plurality of transistors 101 and amplified power is output from the output terminal 104. The matching circuits 102 provide input/output impedance matching for the transistors 101.
In designing of the amplifier, usually, the circuit configurations of the matching circuits 102 are determined so as to conform to the characteristic of the transistors 101 so that the transistors 101 can fully exercise their capabilities. Desirably, the layout of the elements on the substrate constituting each matching circuit 102 is designed properly to reduce the circuit area.
FIG. 8B is a graph of an exemplary maximum available gain (MAG) characteristic of each transistor 101, in which the horizontal axis represents the frequency. In a low-frequency range where each transistor 101 does not satisfy a stability condition (stability factor K>1), the maximum available gain cannot be defined. In FIG. 8A, a maximum stable gain (MSG) characteristic is shown as a measure in such a low-frequency range.
In designing the amplifier based on the design diagram of FIG. 8A, the matching circuits 102 are so designed as to have a loss in the low-frequency range where the stability factor K of the transistor 101 is smaller than 1, to thereby establish the stability factor K of the amplifier to be equal to 1 to prevent the amplifier from oscillating. Where the transistors 101 have a characteristic shown in FIG. 8B, the matching circuits 102 are designed to have the characteristic shown in the graph of FIG. 8C, for example. As a result, in a desired frequency range, the characteristic of the transistors 101 and that of the matching circuits 102 compensate each other, whereby a flat gain characteristic is attained for the amplifier as a whole.
However, if it is necessary to attain both the input/output impedance matching (original purpose of the matching circuits) and the gain flattening by compensating the characteristic of the transistors, the designing of the matching circuits becomes very difficult. There is another problem. Whereas the gain of each transistor varies with the frequency, the output power of each transistor is almost constant with respect to the frequency. Therefore, if the matching circuits are designed so that the loss occurring there varies with frequency, the output power of the amplifier cannot be kept constant though its gain can be kept constant. As described above, it is very difficult to control the characteristics of the matching circuits 102 so that the amplifier has a gain and output power that are kept constant in a stable manner.
Incidentally, with the recent increase of demand relating to communication that is particularly remarkable in LMDS (local multipoint distribution service), the need for low-cost MMICs is now increasing. For cost reduction, it is necessary to increase the frequency range of stable operation as well as to properly design a layout to minimize chip area.